INTRODUCTIONThe relative importance of filtration, diffusion, absorption, reaborption, and secretion to the formation of the urine is still an unknown and disputed problem. None of the theories of renal secretion, nor any of their many modifications, satisfactorily meet the requirements in respect to many details of a fair minded majority. An old well-known obstacle is hydronephrosis. This unique condition, particularly with complete ureteral obstruction, raises some doubt of its true relation to the foregoing factors of renal function. So far, no adequate explanation of hydronephrosis has been presented, and this failure might at first be looked on as the very natural result of our ignorance of the factors that control renal secretion itself. But, on the other hand, it is also possible that this most interesting pathologic condition depends on other factors than those concerned simply with the formation of the urine. That the continued formation of urine after